Onslaughts powers were well over that of Omega Level. In Onslaughts first form, the one he had when he was one with Xavier. In this form, Onslaught has shown a wide variety of abilities including nearly limitless strength, and near indestructibility. Onslaught's psionic powers appeared limitless, appearing to surpass even that of Xavier in his right mind. He was especially powerful in the Astral Plane, where he appeared to have god-like abilities. Even Jean Grey's powers were completely useless against him. From what has been seen, he seems to have total mental control over anyone or anything he wishes, ranging from altering one's perception to believe they are someone or somewhere else (he demonstrated this by making Wolverine believe he was a wolf pup, and making Storm think she was a child), to most conceivable forms of telekinesis. Onslaught also possesed many forms of energy projection, all of which at high levels of power.
After fully evolving into his own, separate consciousness and having Professor X ripped from him in the Astral Plane, Onslaught evolved into a second form, less resembling Magneto and looking more like a cybernetic monster. In this form, his powers were increased to virtually godlike levels, including the ability to mass-manipulate (his complete takeover of all the Sentinels), create and reshape matter at will (shown when he created his citadel, and when he created a second sun to destroy Earth), and an apparent omniscience (as evidenced when he easily thwarted an attempt to sneak into his citadel). His physical strength was also shown to be at obscenely high levels; so much so he was able to literally rip the Crystal Gem of Cytorrak out of Juggernaut and throw him across North America and go toe-to-toe in a physical fight with the Hulk, after the Hulk's mind had been blanked and his strength and rage set to incredible levels, using the mental powers of Jean Grey. For a time, Onslaught had these powers while still in his first form; the majority of these came from the absorption of Nate Grey and Franklin Richards. In the end, Onslaught's true body was revealed to be a mass of pure, psionic energy that was immune to all forms of physical attacks.

Onslaught was a sentient psionic entity created from the consciousness of two mutants - Professor Charles Xavier, telepathic founder of the heroic X-Men, and Magneto, self-styled Master of Magnetism. During a battle between the X-Men and Magneto's Acolytes, Xavier used his telepathic powers to shut down Magneto's mind, rendering him catatonic. During the psionic contact, Magneto's anger, grief and lust for vengeance entered Xavier's consciousness, combining with every long-suppressed negative feeling Xavier had endured during the last 30 years. This aggregation resulted in the being known as Onslaught.
Onslaught kidnapped Franklin Richards, Nate Grey (X-Man), and Professor Xavier, and proceeded to terrorize New York. Then, in one final attack, many of Earth's heroes sacrificed themselves to bring him down. These heroes were really sent into a pocket reality created by Franklin Richards, and came back shortly afterwards.